Recently there has been an increase in the use of monoclonal antibodies for diagnostic and therapeutic applications in vivo. Successful attempts have been made to locate tumor lesions of size &gt;5 mm using radiolabeled monoclonal antibodies and gamma camera imaging. It was hypothesized that in most of these cases specific antigen binding at the tumor site was responsible for the localization of radiolabeled antibody.
In 1988 Rubin et al. discovered the use of radiolabeled, non-specific polyclonal human immunoglobulin for the detection of focal inflammation by scintigraphy (Rubin, R. H., Fischman, A. J., Callahan, R. J. et al., "In(111) Labeled Nonspecific Immunoglobulin Scanning in the Detection of Focal Infection", N. Eng. J. Med., 1989, 30:385-389). It was shown that In(111)-IgG was superior to other radiopharmaceuticals such as Ga(67)-citrate and In-111 labeled white blood cells. Several other investigators have also found that radiolabeled polyclonal IgG localizes well in infectious foci.
The reason for localization of these radiopharmaceuticals in infection is not well understood. In the case of Ga(67)-citrate, the protein leakage of the radiometal may be responsible for the localization of Ga(67)-transferrin at the infectious sites (Tzen, K. Y., Oster, Z. H., Wagner, H. N., et al., "Role of Iron-Binding Proteins and Enhanced Vascular Permeability in the Accumulation of Gallium-67", Journal of Nuclear Medicine, 1980:21, 31-35.) Rubin et al. had considerable success using radiolabeled human polyclonal IgG antibodies in clinical trials. They further showed that Fab fragments of IgG did not localize to the infectious sites, whereas IgG and Fc fragments did. They also reported that the localization of In(111) labeled IgG substantially exceeded the localization of other compounds such as TC(99) labeled human serum albumin and Ga(67)-citrate (Fischman, A. L., Rubin, R. H., White, J. A., et al. "Localization of Fc and Fab Fragment of Nonspecific Polyclonal IgG at Focal Sites of Inflammation," J. Nucl. Med., 1990:31, 1199-1205). The blood clearance half-times (t.sub.1/2) in hours were:
IgG--36.4 PA1 Fc--32.5 PA1 1/2Fc--22.3 PA1 Fab--12.8
Because of the fast clearance times one would expect 1/2Fc and/or Fab to be superior to IgG or Fc for imaging infections. However, animal experiments clearly showed that 1/2Fc and Fab fragments are relatively poor infection imaging agents.